From 40f91ece25fa477eb8869a2218a19f48d5c97705 Mon Sep 17 00:00:00 2001
From: Tapajyoti Bose <44058757+ruppysuppy@users.noreply.github.com>
Date: Fri, 26 Jun 2020 21:02:40 +0530
Subject: [PATCH] Added n-queen (#206)

---
 back-tracking/NQueen.js | 65 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)
 create mode 100644 back-tracking/NQueen.js

diff --git a/back-tracking/NQueen.js b/back-tracking/NQueen.js
new file mode 100644
index 000000000..dfd3dfa9f
--- /dev/null
+++ b/back-tracking/NQueen.js
@@ -0,0 +1,65 @@
+class NQueen {
+  constructor (size) {
+    this.board = new Array(size).fill('.').map(() => new Array(size).fill('.'))
+    this.size = size
+  }
+
+  isValid ([row, col]) {
+    // function to check if the placement of the queen in the given location is valid
+
+    // checking the left of the current row
+    for (let i = 0; i < col; i++) {
+      if (this.board[row][i] === 'Q') return false
+    }
+
+    // checking the upper left diagonal
+    for (let i = row, j = col; i >= 0 && j >= 0; i--, j--) {
+      if (this.board[i][j] === 'Q') return false
+    }
+
+    // checking the lower left diagonal
+    for (let i = row, j = col; j >= 0 && i < this.size; i++, j--) {
+      if (this.board[i][j] === 'Q') return false
+    }
+
+    return true
+  }
+
+  solve (col = 0) {
+    // function to solve the board
+    if (col >= this.size) { return true }
+
+    for (let i = 0; i < this.size; i++) {
+      if (this.isValid([i, col])) {
+        this.board[i][col] = 'Q'
+
+        if (this.solve(col + 1)) { return true }
+
+        // backtracking
+        this.board[i][col] = '.'
+      }
+    }
+
+    return false
+  }
+
+  printBoard () {
+    // utility function to display the board
+    for (const row of this.board) {
+      console.log(...row)
+    }
+  }
+}
+
+function main () {
+  const board = new NQueen(8)
+
+  board.printBoard()
+  console.log('\n')
+
+  board.solve()
+
+  board.printBoard()
+}
+
+main()